[chrisdarl]

Due to ongoing gear issues ( 2013/14 (newer shape) i30 - Clutch / Gear issues) my next attempt is to replace the master cylinder as a lot of air came out of the oil last time, the oil change gave me a good few months of symptom-free driving so hopeful it's this and not a whole not clutch kit needed.

I remember last time when I drained the oil that one of the pipes that connected to the rear of the master cylinder had a small o-ring on it that inserted into the master cylinder.

Can anyone advise what type of o-ring I should use? I'm using DOT-4 oil. If nobody knows the size some advice of the type of material would be helpful, I have a set of Nitril o-rings but not sure if they would be suitable?

Thank you in advance, everyone!

[chrisdarl]

Just to add... the o-ring is the one shown in this picture (part no: 41690-2E005 I think)

[nzenigma]

"I remember last time when I drained the oil that one of the pipes that connected to the rear of the master cylinder had a small o-ring on it that inserted into the master cylinder."

I assume and hope you really mean fluid, NOT OIL!

No idea about the 'O' ring, you have one, take it off and check the size. Usually one of 3  sizes.

Also unclear why you are pulling master apart and draining the fluid to remove the air. Just bleed at the slave cylinder, or reverse bleed at same slave nipple.

[chrisdarl]

Apologies, yes, hydraulic fluid not oil.

Pulled the master out as I bled the system before and the symptoms returned after a few months, so bleeding didn't permanently resolve the issue. I have replaced the master cylinder today with a new one and bleed the system.

Sadly, doesn't look like it's solved the issue and I'm guessing a new clutch is needed. What are your thoughts? Any particular brands I should go for or avoid?

Here in the UK, a garage has offered Asin or Blueprint manufactured clutch kits.

[nzenigma]

Down here , the Exedy brand is most highly rated; but many others are good too.

But... from the information you have supplied so far, I think you only have one oar in the water. 

"....the oil (sic) change gave me a good few months of symptom-free driving ..."
If your clutch is weak or worn, it stays that way. No Easter resurrection, irrespective of any devotional expectation. 

"...a lot of air came out ..."
a defective clutch ( pressure and driven plate) cannot force air into the hydraulic system. 'Hydraulic' is where you problem lies.

If you have a new master, no fluid leaks, and have correctly bled the system ( reverse bleed?), there should be a firm pedal.